One of the most requested features for Gmail is push support for the Apple iPhone. Google announced yesterday that they now support push Gmail on the iPhone through Google Sync. To get started with this, go here and read the various instructions.
We knew this would be coming because a Googler told us back in March it would be here soon. And trust me, people are delighted it is here. There are literally dozens and dozens of threads at the Google Mobile iPhone Forum on this topic.
Personally, I really do not want Push notifications for my email. I just don't want my iPhone beeping and vibrating every other second. Plus, I am sure it won't do good for my battery life. Personally, I know when I check email on my iPhone that there will be email waiting for me - I don't need to be pushed this on me. I can see for email accounts that get fewer emails, this may make sense.
Forum discussion at Google Mobile iPhone Forum.